The present invention is a device used to generate a gaseous plasma in which an oscillating electromagnetic field ionizes neutral species in the gas phase to form ions and electrons, excites neutral species to form electronically excited atoms and molecules, and dissociates molecules to form atoms and radicals. Industrial plasma processing involves the exposure of a workpiece to the plasma effluents in order to remove material from the substrate surface (etching), grow material on the substrate surface (deposition), chemically alter the surface (plasma oxidation, nitration, surface cleaning and passivation), physically modify the surface (surface roughing or smoothing), or generally modify the conditions on the surface or in the body of the workpiece (e.g. plasma sterilization). Another application is gas abatement which involves using the plasma source as a low-temperature incinerator to convert toxic or environmentally harmful industrial emissions into harmless substances. The utility of a plasma source relates for many applications to the density of charged particles in the plasma, ions and electrons. This density, in turn, is governed by the rate of production versus the rate of loss of ions and electrons. Plasmas and plasma sources are characterized and differentiated not only by the density of charged particles in the resulting plasma but in addition by the frequency of the generating electromagnetic field and by the range of gas pressure or vacuum suitable for its operation. The following review of prior art is limited to the technology associated with plasmas generated by microwaves of frequency 2.45 GHz.
ECR. In a plasma electrons oscillate with the electrical component of the generating electromagnetic field. The electrical as well as the magnetic vector is located in a plane perpendicular to the direction of propagation of the electromagnetic field. If a strong magnetic field is imposed on the plasma the electrons are forced to gyrate around the magnetic field lines with a rotation frequency determined by the strength of the magnetic field. The plane of rotation is perpendicular to the magnetic field lines. By proper adjustment of the electromagnetic frequency and magnetic strength the oscillation frequency due to the electromagnetic field and the gyration frequency caused by the superimposed magnetic field can be brought to coincide. This defines the special condition of electron cyclotron resonance, ECR. In addition to the matching of frequencies the spacial arrangement must be such that the electrical vector of the electromagnetic field is in the plane of gyration. This is the case when the microwave propagation is parallel with the magnetic field lines. The ECR condition is characterized by a drastically increased microwave power absorption by the plasma electrons resulting in much higher densities of charged particles and a much greater degree of dissociation in an ECR plasma than in a microwave plasma without the auxiliary magnetic field. Collisional scattering of electrons by neutrals will interfere with the electron gyration resulting in dampening of the effect as the gas pressure is raised. However, the effect is still substantial in helium at 1 torr as reported by B. Lax, P. Allis, and S. C. Brown, J. Appl. Phys., 21, 1297 (1950).
Electromagnets. In practice, the necessary magnetic field can be produced either by an electromagnet or by a permanent magnet. By far the most common has been to use electromagnets. These magnets are usually in the shape of a solenoid which encloses the process chamber or part of the chamber, the plasma generating subchamber. In order to achieve adequate control of the shape of the magnetic field two or even three solenoid coils are used. This is exemplified by U.S. Pat. Nos. 4,876,983, 4,915,979, and 4,970,435. In order to produce a magnetic field of the proper strength currents in excess of 100 amps are needed in the coils producing heat and demanding elaborate cooling efforts. In addition, the circuitry to control the strength of the magnetic fields generated by the coils as well as the considerable size and weight of such electromagnets increase the cost of these systems very significantly. The size, or "footprint", is of particular concern in the semiconductor industry, where cleanroom space is at a premium.
Permanent magnets. Permanent magnets have been used in order to avoid the costly disadvantages of electromagnets. The problem now becomes one of placing the magnets sufficiently close to the plasma and the workpiece considering the rapid decay of the magnetic field strength with the distance from the magnet surface. In U.S. Pat. No. 4,433,228 the permanent magnet is placed in the microwave waveguide itself. While this arrangement brings the magnet very close to the workpiece it necessitates that the microwaves pass through the magnetic material thereby limiting the microwave power that can be applied in order to avoid destroying the magnet by the generated heat. In addition, the electromagnetic field of the microwave is perturbed by passage through the magnetic material. This disadvantage is avoided in U.S. Pat. No. 5,196,670 where the microwaves are brought in between the magnet and the quartz window allowing the microwaves to pass into the chamber without passing through the magnet. However, this effectively moves the magnet further away from the preferable location of the ECR plane in the chamber which, in turn, necessitates a considerably more powerful and costly magnet. Permanent magnets have also been used in connection with high density plasmas in order to reduce the rate of loss of electrons to the chamber walls by magnetic confinement. Here, the magnets function by repelling the electrons away from the walls back into the plasma. This is illustrated by U.S. Pat. No. 4,483,737, where the plasma source is a hot filament, and by U.S. Pat. No. 5,032,202, where the source is an electromagnetic ECR subchamber. In U.S. Pat. No. 5,032,205 permanent magnets provide the necessary magnetic field for ECR operation and the plasma source is an RF electrode in the chamber itself. A similar setup is described in U.S. Pat. No. 4,745,337, where the in-chamber electrodes are microwave antennas.
Remote processing. Remote processing here designates treatment of a substrate located outside the plasma excitation region in a separate, downstream processing chamber as opposed to and distinct from the in situ plasma generation chamber. There are a variety of industrial processes that involve the plasma activation of a gas or gas mixture, transport of the activated gas effluent to a downstream region, and reaction to deposit a film on a substrate, to remove or etch a surface layer from a substrate, or to chemically or physically alter or modify the surface or body of the substrate. The gas or gas mixture can be activated by a number of means such as a hot filament, a microwave discharge, a DC or RF discharge, and plasma jets or torches. U.S. Pat. No. 5,206,471 describes a microwave activated gas generator, in which the gas is passed through the MW waveguide in a quartz tube, but with no provisions for creation of ECR conditions and thus much less efficient. Another example is U.S. Pat. No. 5,115,166 using a plurality of similar microwave plasma generators, again unsuitable for ECR operation, employing the downstream processing region for substrate sterilization.
There is no prior art in the technology area known as reactive sputter deposition of optical thin films closely related to the present invention. Typically, a substrate is moved from a sputtering zone with an inert atmosphere, where the substrate is coated with a metal or metal alloy, to a reaction zone with a reactive and/or activated atmosphere, where the sputtered material is chemically altered to form the final film. The sputter zone is separated from the reaction zone by either physical means, as in U.S. Pat. No. 4,420,385, or by formation of concentration gradients of the proper chemicals, as in U.S. Pat. No. 4,851,095. Remote plasma activation of the gases flowing to the reaction zone is expected to accelerate the conversion of the sputtered film to the final, optically transparent film.
The design for in situ plasma sources has been greatly restricted by the need to make room for the workpiece in the process chamber. This concern has prevented the ideal design which would have the microwave field enter from one side of the chamber and a permanent magnet located on the opposite side thereby preserving the necessary parallelism between the magnetic field lines and microwave propagation. This design enables the magnet to be much closer to the plasma so that the necessary magnetic field can be achieved with a much smaller and less costly magnet. The present invention makes this ideal or optimal design possible by moving the workpiece completely out of the source chamber. Remote plasma sources are usually under restrictions too severe to allow for the cost and bulk of the traditional electromagnetic ECR source. The savings in production cost and in space requirements associated with the present invention will for the first time make a remote ECR plasma source production worthy.
Gas abatement. Release of gases that are toxic to humans or generally harmful to the global environment is of growing concern to the Environmental Protection Agency and to the industrial producers of these gaseous emissions. The semiconductor industry is particularly affected by this concern as the fabrication of computer chips involves very toxic chemicals (arsine, phosphine, chlorine) as well as very stable compounds capable of reaching the upper atmosphere inflicting serious and long-term damage to the planetary climate: ozone depletion by chlorofluorocarbons and global warming by perfluorinated compounds. The present invention is thought to be especially suited for abatement of the perfluorinated compounds, CF.sub.4, C.sub.2 F.sub.6, and NF.sub.3, used in thin film etching and cleaning of chambers for chemical vapor deposition (CVD). The application of the present invention for gas abatement involves location of the plasma source downstream from a processing chamber. The gas molecules in the effluent of the processing chamber are dissociated by electron impact collisions in the plasma, and suitable reaction partners for the molecular fragments are added either just before or right after passage through the plasma source.
There does not seem to be any consideration of using plasma abatement for incineration of industrial emissions prior to the recent concern for global warming by perfluorinated compounds. Thus, the prior art for this application is limited to three reports at conferences this year involving, in all cases, non-ECR plasma abatement. The proof-of-principle report is provided by F. W. Breitbarth, H. J. Tiller, and K. Dumke, Proceedings of the 11th International Symposium on Plasma Chemistry, 728 (1993). They demonstrate abatement of C.sub.4 F.sub.8 and CHF.sub.3 in a capacitively coupled RF discharge. Demonstration of microwave plasma abatement of C.sub.2 F.sub.6 was provided in a report by M. T. Mocella, V. Mohindra, and H. H. Sawin at the meeting of The Electrochemical Society, San Francisco, May 1994. An additional report on microwave abatement of C.sub.2 F.sub.6 was given by J. D. Cripe at the Global Warming Symposium, Dallas, June 1994. The most significant feature separating the present invention from all reported abatement experiments is the ability to operate in the ECR mode afforded by the permanent magnet. For the application for gas abatement the ECR feature has particular significance as it implies a higher electron density in the plasma, probably by a factor of 10 to 100, which Should affect the efficiency in direct proportion. The mechanism of gas abatement by a plasma clearly is based on extensive dissocoation of the gas by electron impact collisions and therefore depends on the availability of electrons. Another property of ECR plasmas of special importance to gas abatement is the drastically increased power absorption in the ECR mode. High flow gas abatement, e.g. 2 standard liters per minute of C.sub.2 F.sub.6, is expected to require input of 2-5 kWatts power to the plasma, which is far beyond the capability of a non-ECR plasma to absorbe. In addition, there are specific differences between the present invention and each of the reported experiments Thus, it is widely recognized that the microwave frequency used in the present invention, 2.45 GHz, is much more efficient for the purpose of plasma dissociation than the RF frequency, 13.56 MHz or slower, used by Breitbarth et al Likewise, the experimental setup employed by Mocella et al is a socalled surface-wave microwave launcher in which the abatement gas is guided by a quartz tube through the microwave waveguide. A serious problem is associated with the fluorine atoms created in the plasma inside the quartz tubing as they will react with and erode the quartz or any other suitable material which can be fashioned into tubing.